During the harvesting of forage materials it is customary to transfer the harvested materials from the harvester itself to a truck or wagon which is driven alongside of the harvester while the operation is continuing. When the truck becomes full, however, it is necessary for the harvester to then accumulate and hold the materials on board until the truck has emptied its load and has reappeared in position for receiving the next supply, in the event that harvesting is to be carried out nonstop, which, of course, is quite desirable. In view of the fact that the time when the truck is not available may be substantial, it is desirable to maximize the holding capacity of the harvester in order that field operations need not be temporarily halted while return of the truck is awaited. However, governmental height and width regulations with respect to over-the-road vehicles have a bearing on and restrict the dimensions which can be assumed by the box. Moreover, there are problems in properly filling upright, straight sided box containers when blowing in crop materials from a front opening thereof, as well as problems in conveyor unloading from such boxes. Furthermore, unloading from the bottom of the box, yet placing the point of discharge into the truck or other receiving vehicle at a point which is sufficiently high to avoid interference with the adjacent vehicle is also a significant problem to be dealt with.